It is known to extrude a molten mass of the thermoplastic material into filaments, to lead the filaments through a liquid coolant for stiffening them, and to cut the stiffened filaments into short cylindrical stumps or chips by means of shears. However, because of the softness of the warm extruded material, and due to non-uniformity of the sizes of the holes in the die and of the cooling rates of the several filaments, differences in length are bound to arise among the filaments. These differences are either absorbed in the cutting step, thus leading to non-uniformity in the size and shape of the chips, or form loops of increasing length, thus causing malfunctions of the chip-making machine, which malfunctions eventually have to be cured by stopping the machine for removing the loops.
There have been attempts in the past to overcome the above inconveniences by avoiding the cooling step and cutting the filaments into chips directly at the exit from the die. These attempts, however, have only been successful with quickly stiffening materials such as polyvinyl chloride. With materials such as polyethylene and polypropylene, on the other hand, the chips, if cut directly at the exit of the die, are soft and sticky and tend to lump together and stick to the shearing blade and to the walls of the surrounding enclosure.
This invention therefore has the object of providing a process for making chips of thermoplastic material such as polyethylene or polypropylene and the like, by means of which the cooling step through a liquid coolant may be dispensed with without incurring in the above mentioned drawbacks.
A further object of the invention is to achieve a high degree of uniformity in the manufacture of chips of thermoplastic material such as polyethylene and polypropylene.
A further object is to provide a chip-making apparatus of the above mentioned kind, which is more compact and more easily operated than prior apparatus.